ecp_nistz256.c

/******************************************************************************
 *                                                                            *
 * Copyright 2014 Intel Corporation                                           *
 *                                                                            *
 * Licensed under the Apache License, Version 2.0 (the "License");            *
 * you may not use this file except in compliance with the License.           *
 * You may obtain a copy of the License at                                    *
 *                                                                            *
 *    http://www.apache.org/licenses/LICENSE-2.0                              *
 *                                                                            *
 * Unless required by applicable law or agreed to in writing, software        *
 * distributed under the License is distributed on an "AS IS" BASIS,          *
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.   *
 * See the License for the specific language governing permissions and        *
 * limitations under the License.                                             *
 *                                                                            *
 ******************************************************************************
 *                                                                            *
 * Developers and authors:                                                    *
 * Shay Gueron (1, 2), and Vlad Krasnov (1)                                   *
 * (1) Intel Corporation, Israel Development Center                           *
 * (2) University of Haifa                                                    *
 * Reference:                                                                 *
 * S.Gueron and V.Krasnov, "Fast Prime Field Elliptic Curve Cryptography with *
 *                          256 Bit Primes"                                   *
 *                                                                            *
 ******************************************************************************/

#include <string.h>

#include <openssl/bn.h>
#include <openssl/err.h>
#include <openssl/ec.h>
#include "cryptlib.h"

#include "ec_lcl.h"

#if BN_BITS2 != 64
# define TOBN(hi,lo)	lo,hi
#else
# define TOBN(hi,lo)	((BN_ULONG)hi<<32|lo)
#endif

#if defined(__GNUC__)
# define ALIGN32	__attribute((aligned(32)))
#elif defined(_MSC_VER)
# define ALIGN32	__declspec(align(32))
#else
# define ALIGN32
#endif

#define ALIGNPTR(p,N)	((unsigned char *)p+N-(size_t)p%N)
#define P256_LIMBS	(256/BN_BITS2)

typedef unsigned short u16;

typedef struct {
    BN_ULONG X[P256_LIMBS];
    BN_ULONG Y[P256_LIMBS];
    BN_ULONG Z[P256_LIMBS];
} P256_POINT;

typedef struct {
    BN_ULONG X[P256_LIMBS];
    BN_ULONG Y[P256_LIMBS];
} P256_POINT_AFFINE;

typedef P256_POINT_AFFINE PRECOMP256_ROW[64];

/* structure for precomputed multiples of the generator */
typedef struct ec_pre_comp_st {
    const EC_GROUP *group;      /* Parent EC_GROUP object */
    size_t w;                   /* Window size */
    /* Constant time access to the X and Y coordinates of the pre-computed,
     * generator multiplies, in the Montgomery domain. Pre-calculated
     * multiplies are stored in affine form. */
    PRECOMP256_ROW *precomp;
    void *precomp_storage;
    int references;
} EC_PRE_COMP;

/* Functions implemented in assembly */
/* Modular mul by 2: res = 2*a mod P */
void ecp_nistz256_mul_by_2(BN_ULONG res[P256_LIMBS],
                           const BN_ULONG a[P256_LIMBS]);
/* Modular div by 2: res = a/2 mod P */
void ecp_nistz256_div_by_2(BN_ULONG res[P256_LIMBS],
                           const BN_ULONG a[P256_LIMBS]);
/* Modular mul by 3: res = 3*a mod P */
void ecp_nistz256_mul_by_3(BN_ULONG res[P256_LIMBS],
                           const BN_ULONG a[P256_LIMBS]);
/* Modular add: res = a+b mod P	  */
void ecp_nistz256_add(BN_ULONG res[P256_LIMBS],
                      const BN_ULONG a[P256_LIMBS],
                      const BN_ULONG b[P256_LIMBS]);
/* Modular sub: res = a-b mod P	  */
void ecp_nistz256_sub(BN_ULONG res[P256_LIMBS],
                      const BN_ULONG a[P256_LIMBS],
                      const BN_ULONG b[P256_LIMBS]);
/* Modular neg: res = -a mod P	  */
void ecp_nistz256_neg(BN_ULONG res[P256_LIMBS], const BN_ULONG a[P256_LIMBS]);
/* Montgomery mul: res = a*b*2^-256 mod P */
void ecp_nistz256_mul_mont(BN_ULONG res[P256_LIMBS],
                           const BN_ULONG a[P256_LIMBS],
                           const BN_ULONG b[P256_LIMBS]);
/* Montgomery sqr: res = a*a*2^-256 mod P */
void ecp_nistz256_sqr_mont(BN_ULONG res[P256_LIMBS],
                           const BN_ULONG a[P256_LIMBS]);
/* Convert a number from Montgomery domain, by multiplying with 1 */
void ecp_nistz256_from_mont(BN_ULONG res[P256_LIMBS],
                            const BN_ULONG in[P256_LIMBS]);
/* Convert a number to Montgomery domain, by multiplying with 2^512 mod P*/
void ecp_nistz256_to_mont(BN_ULONG res[P256_LIMBS],
                          const BN_ULONG in[P256_LIMBS]);
/* Functions that perform constant time access to the precomputed tables */
void ecp_nistz256_select_w5(P256_POINT * val,
                            const P256_POINT * in_t, int index);
void ecp_nistz256_select_w7(P256_POINT_AFFINE * val,
                            const P256_POINT_AFFINE * in_t, int index);

/* One converted into the Montgomery domain */
static const BN_ULONG ONE[P256_LIMBS] = {
    TOBN(0x00000000, 0x00000001), TOBN(0xffffffff, 0x00000000),
    TOBN(0xffffffff, 0xffffffff), TOBN(0x00000000, 0xfffffffe)
};

static void *ecp_nistz256_pre_comp_dup(void *);
static void ecp_nistz256_pre_comp_free(void *);
static void ecp_nistz256_pre_comp_clear_free(void *);
static EC_PRE_COMP *ecp_nistz256_pre_comp_new(const EC_GROUP * group);

/* Precomputed tables for the default generator */
#include "ecp_nistz256_table.c"

/* Recode window to a signed digit, see ecp_nistputil.c for details */
static unsigned int _booth_recode_w5(unsigned int in)
{
    unsigned int s, d;

    s = ~((in >> 5) - 1);
    d = (1 << 6) - in - 1;
    d = (d & s) | (in & ~s);
    d = (d >> 1) + (d & 1);

    return (d << 1) + (s & 1);
}

static unsigned int _booth_recode_w7(unsigned int in)
{
    unsigned int s, d;

    s = ~((in >> 7) - 1);
    d = (1 << 8) - in - 1;
    d = (d & s) | (in & ~s);
    d = (d >> 1) + (d & 1);

    return (d << 1) + (s & 1);
}

static void copy_conditional(BN_ULONG dst[P256_LIMBS],
                             const BN_ULONG src[P256_LIMBS], BN_ULONG move)
{
    BN_ULONG mask1 = -move;
    BN_ULONG mask2 = ~mask1;

    dst[0] = (src[0] & mask1) ^ (dst[0] & mask2);
    dst[1] = (src[1] & mask1) ^ (dst[1] & mask2);
    dst[2] = (src[2] & mask1) ^ (dst[2] & mask2);
    dst[3] = (src[3] & mask1) ^ (dst[3] & mask2);
    if (P256_LIMBS == 8) {
        dst[4] = (src[4] & mask1) ^ (dst[4] & mask2);
        dst[5] = (src[5] & mask1) ^ (dst[5] & mask2);
        dst[6] = (src[6] & mask1) ^ (dst[6] & mask2);
        dst[7] = (src[7] & mask1) ^ (dst[7] & mask2);
    }
}

static BN_ULONG is_zero(BN_ULONG in)
{
    in |= (0 - in);
    in = ~in;
    in &= BN_MASK2;
    in >>= BN_BITS2 - 1;
    return in;
}

static BN_ULONG is_equal(const BN_ULONG a[P256_LIMBS],
                         const BN_ULONG b[P256_LIMBS])
{
    BN_ULONG res;

    res = a[0] ^ b[0];
    res |= a[1] ^ b[1];
    res |= a[2] ^ b[2];
    res |= a[3] ^ b[3];
    if (P256_LIMBS == 8) {
        res |= a[4] ^ b[4];
        res |= a[5] ^ b[5];
        res |= a[6] ^ b[6];
        res |= a[7] ^ b[7];
    }

    return is_zero(res);
}

static BN_ULONG is_one(const BN_ULONG a[P256_LIMBS])
{
    BN_ULONG res;

    res = a[0] ^ ONE[0];
    res |= a[1] ^ ONE[1];
    res |= a[2] ^ ONE[2];
    res |= a[3] ^ ONE[3];
    if (P256_LIMBS == 8) {
        res |= a[4] ^ ONE[4];
        res |= a[5] ^ ONE[5];
        res |= a[6] ^ ONE[6];
    }

    return is_zero(res);
}

#ifndef ECP_NISTZ256_REFERENCE_IMPLEMENTATION
void ecp_nistz256_point_double(P256_POINT * r, const P256_POINT * a);
void ecp_nistz256_point_add(P256_POINT * r,
                            const P256_POINT * a, const P256_POINT * b);
void ecp_nistz256_point_add_affine(P256_POINT * r,
                                   const P256_POINT * a,
                                   const P256_POINT_AFFINE * b);
#else
/* Point double: r = 2*a */
static void ecp_nistz256_point_double(P256_POINT * r, const P256_POINT * a)
{
    BN_ULONG S[P256_LIMBS];
    BN_ULONG M[P256_LIMBS];
    BN_ULONG Zsqr[P256_LIMBS];
    BN_ULONG tmp0[P256_LIMBS];

    const BN_ULONG *in_x = a->X;
    const BN_ULONG *in_y = a->Y;
    const BN_ULONG *in_z = a->Z;

    BN_ULONG *res_x = r->X;
    BN_ULONG *res_y = r->Y;
    BN_ULONG *res_z = r->Z;

    ecp_nistz256_mul_by_2(S, in_y);

    ecp_nistz256_sqr_mont(Zsqr, in_z);

    ecp_nistz256_sqr_mont(S, S);

    ecp_nistz256_mul_mont(res_z, in_z, in_y);
    ecp_nistz256_mul_by_2(res_z, res_z);

    ecp_nistz256_add(M, in_x, Zsqr);
    ecp_nistz256_sub(Zsqr, in_x, Zsqr);

    ecp_nistz256_sqr_mont(res_y, S);
    ecp_nistz256_div_by_2(res_y, res_y);

    ecp_nistz256_mul_mont(M, M, Zsqr);
    ecp_nistz256_mul_by_3(M, M);

    ecp_nistz256_mul_mont(S, S, in_x);
    ecp_nistz256_mul_by_2(tmp0, S);

    ecp_nistz256_sqr_mont(res_x, M);

    ecp_nistz256_sub(res_x, res_x, tmp0);
    ecp_nistz256_sub(S, S, res_x);

    ecp_nistz256_mul_mont(S, S, M);
    ecp_nistz256_sub(res_y, S, res_y);
}

/* Point addition: r = a+b */
static void ecp_nistz256_point_add(P256_POINT * r,
                                   const P256_POINT * a, const P256_POINT * b)
{
    BN_ULONG U2[P256_LIMBS], S2[P256_LIMBS];
    BN_ULONG U1[P256_LIMBS], S1[P256_LIMBS];
    BN_ULONG Z1sqr[P256_LIMBS];
    BN_ULONG Z2sqr[P256_LIMBS];
    BN_ULONG H[P256_LIMBS], R[P256_LIMBS];
    BN_ULONG Hsqr[P256_LIMBS];
    BN_ULONG Rsqr[P256_LIMBS];
    BN_ULONG Hcub[P256_LIMBS];

    BN_ULONG res_x[P256_LIMBS];
    BN_ULONG res_y[P256_LIMBS];
    BN_ULONG res_z[P256_LIMBS];

    BN_ULONG in1infty, in2infty;

    const BN_ULONG *in1_x = a->X;
    const BN_ULONG *in1_y = a->Y;
    const BN_ULONG *in1_z = a->Z;

    const BN_ULONG *in2_x = b->X;
    const BN_ULONG *in2_y = b->Y;
    const BN_ULONG *in2_z = b->Z;

    /* We encode infinity as (0,0), which is not on the curve,
     * so it is OK. */
    in1infty = in1_x[0] | in1_x[1] | in1_x[2] | in1_x[3] |
               in1_y[0] | in1_y[1] | in1_y[2] | in1_y[3];
    if (P256_LIMBS == 8)
        in1infty |= in1_x[4] | in1_x[5] | in1_x[6] | in1_x[7] |
                    in1_y[4] | in1_y[5] | in1_y[6] | in1_y[7];

    in2infty = in2_x[0] | in2_x[1] | in2_x[2] | in2_x[3] |
               in2_y[0] | in2_y[1] | in2_y[2] | in2_y[3];
    if (P256_LIMBS == 8)
        in2infty |= in2_x[4] | in2_x[5] | in2_x[6] | in2_x[7] |
                    in2_y[4] | in2_y[5] | in2_y[6] | in2_y[7];

    in1infty = is_zero(in1infty);
    in2infty = is_zero(in2infty);

    ecp_nistz256_sqr_mont(Z2sqr, in2_z);        /* Z2^2 */
    ecp_nistz256_sqr_mont(Z1sqr, in1_z);        /* Z1^2 */

    ecp_nistz256_mul_mont(S1, Z2sqr, in2_z);    /* S1 = Z2^3 */
    ecp_nistz256_mul_mont(S2, Z1sqr, in1_z);    /* S2 = Z1^3 */

    ecp_nistz256_mul_mont(S1, S1, in1_y);       /* S1 = Y1*Z2^3 */
    ecp_nistz256_mul_mont(S2, S2, in2_y);       /* S2 = Y2*Z1^3 */
    ecp_nistz256_sub(R, S2, S1);                /* R = S2 - S1 */

    ecp_nistz256_mul_mont(U1, in1_x, Z2sqr);    /* U1 = X1*Z2^2 */
    ecp_nistz256_mul_mont(U2, in2_x, Z1sqr);    /* U2 = X2*Z1^2 */
    ecp_nistz256_sub(H, U2, U1);                /* H = U2 - U1 */

    /* This should not happen during sign/ecdh,
     * so no constant time violation */
    if (is_equal(U1, U2) && !in1infty && !in2infty) {
        if (is_equal(S1, S2)) {
            ecp_nistz256_point_double(r, a);
            return;
        } else {
            memset(r, 0, sizeof(*r));
            return;
        }
    }

    ecp_nistz256_sqr_mont(Rsqr, R);             /* R^2 */
    ecp_nistz256_mul_mont(res_z, H, in1_z);     /* Z3 = H*Z1*Z2 */
    ecp_nistz256_sqr_mont(Hsqr, H);             /* H^2 */
    ecp_nistz256_mul_mont(res_z, res_z, in2_z); /* Z3 = H*Z1*Z2 */
    ecp_nistz256_mul_mont(Hcub, Hsqr, H);       /* H^3 */

    ecp_nistz256_mul_mont(U2, U1, Hsqr);        /* U1*H^2 */
    ecp_nistz256_mul_by_2(Hsqr, U2);            /* 2*U1*H^2 */

    ecp_nistz256_sub(res_x, Rsqr, Hsqr);
    ecp_nistz256_sub(res_x, res_x, Hcub);

    ecp_nistz256_sub(res_y, U2, res_x);

    ecp_nistz256_mul_mont(S2, S1, Hcub);
    ecp_nistz256_mul_mont(res_y, R, res_y);
    ecp_nistz256_sub(res_y, res_y, S2);

    copy_conditional(res_x, in2_x, in1infty);
    copy_conditional(res_y, in2_y, in1infty);
    copy_conditional(res_z, in2_z, in1infty);

    copy_conditional(res_x, in1_x, in2infty);
    copy_conditional(res_y, in1_y, in2infty);
    copy_conditional(res_z, in1_z, in2infty);

    memcpy(r->X, res_x, sizeof(res_x));
    memcpy(r->Y, res_y, sizeof(res_y));
    memcpy(r->Z, res_z, sizeof(res_z));
}

/* Point addition when b is known to be affine: r = a+b */
static void ecp_nistz256_point_add_affine(P256_POINT * r,
                                          const P256_POINT * a,
                                          const P256_POINT_AFFINE * b)
{
    BN_ULONG U2[P256_LIMBS], S2[P256_LIMBS];
    BN_ULONG Z1sqr[P256_LIMBS];
    BN_ULONG H[P256_LIMBS], R[P256_LIMBS];
    BN_ULONG Hsqr[P256_LIMBS];
    BN_ULONG Rsqr[P256_LIMBS];
    BN_ULONG Hcub[P256_LIMBS];

    BN_ULONG res_x[P256_LIMBS];
    BN_ULONG res_y[P256_LIMBS];
    BN_ULONG res_z[P256_LIMBS];

    BN_ULONG in1infty, in2infty;

    const BN_ULONG *in1_x = a->X;
    const BN_ULONG *in1_y = a->Y;
    const BN_ULONG *in1_z = a->Z;

    const BN_ULONG *in2_x = b->X;
    const BN_ULONG *in2_y = b->Y;

    /* In affine representation we encode infty as (0,0),
     * which is not on the curve, so it is OK */
    in1infty = in1_x[0] | in1_x[1] | in1_x[2] | in1_x[3] |
               in1_y[0] | in1_y[1] | in1_y[2] | in1_y[3];
    if (P256_LIMBS == 8)
        in1infty |= in1_x[4] | in1_x[5] | in1_x[6] | in1_x[7] |
                    in1_y[4] | in1_y[5] | in1_y[6] | in1_y[7];

    in2infty = in2_x[0] | in2_x[1] | in2_x[2] | in2_x[3] |
               in2_y[0] | in2_y[1] | in2_y[2] | in2_y[3];
    if (P256_LIMBS == 8)
        in2infty |= in2_x[4] | in2_x[5] | in2_x[6] | in2_x[7] |
                    in2_y[4] | in2_y[5] | in2_y[6] | in2_y[7];

    in1infty = is_zero(in1infty);
    in2infty = is_zero(in2infty);

    ecp_nistz256_sqr_mont(Z1sqr, in1_z);        /* Z1^2 */

    ecp_nistz256_mul_mont(U2, in2_x, Z1sqr);    /* U2 = X2*Z1^2 */
    ecp_nistz256_sub(H, U2, in1_x);             /* H = U2 - U1 */

    ecp_nistz256_mul_mont(S2, Z1sqr, in1_z);    /* S2 = Z1^3 */

    ecp_nistz256_mul_mont(res_z, H, in1_z);     /* Z3 = H*Z1*Z2 */

    ecp_nistz256_mul_mont(S2, S2, in2_y);       /* S2 = Y2*Z1^3 */
    ecp_nistz256_sub(R, S2, in1_y);             /* R = S2 - S1 */

    ecp_nistz256_sqr_mont(Hsqr, H);             /* H^2 */
    ecp_nistz256_sqr_mont(Rsqr, R);             /* R^2 */
    ecp_nistz256_mul_mont(Hcub, Hsqr, H);       /* H^3 */

    ecp_nistz256_mul_mont(U2, in1_x, Hsqr);     /* U1*H^2 */
    ecp_nistz256_mul_by_2(Hsqr, U2);            /* 2*U1*H^2 */

    ecp_nistz256_sub(res_x, Rsqr, Hsqr);
    ecp_nistz256_sub(res_x, res_x, Hcub);
    ecp_nistz256_sub(H, U2, res_x);

    ecp_nistz256_mul_mont(S2, in1_y, Hcub);
    ecp_nistz256_mul_mont(H, H, R);
    ecp_nistz256_sub(res_y, H, S2);

    copy_conditional(res_x, in2_x, in1infty);
    copy_conditional(res_x, in1_x, in2infty);

    copy_conditional(res_y, in2_y, in1infty);
    copy_conditional(res_y, in1_y, in2infty);

    copy_conditional(res_z, ONE, in1infty);
    copy_conditional(res_z, in1_z, in2infty);

    memcpy(r->X, res_x, sizeof(res_x));
    memcpy(r->Y, res_y, sizeof(res_y));
    memcpy(r->Z, res_z, sizeof(res_z));
}
#endif

/* r = in^-1 mod p */
static void ecp_nistz256_mod_inverse(BN_ULONG r[P256_LIMBS],
                                     const BN_ULONG in[P256_LIMBS])
{
    /* The poly is ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff
       We use FLT and used poly-2 as exponent */
    BN_ULONG p2[P256_LIMBS];
    BN_ULONG p4[P256_LIMBS];
    BN_ULONG p8[P256_LIMBS];
    BN_ULONG p16[P256_LIMBS];
    BN_ULONG p32[P256_LIMBS];
    BN_ULONG res[P256_LIMBS];
    int i;

    ecp_nistz256_sqr_mont(res, in);
    ecp_nistz256_mul_mont(p2, res, in);         /* 3*p */

    ecp_nistz256_sqr_mont(res, p2);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(p4, res, p2);         /* f*p */

    ecp_nistz256_sqr_mont(res, p4);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(p8, res, p4);         /* ff*p */

    ecp_nistz256_sqr_mont(res, p8);
    for (i = 0; i < 7; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(p16, res, p8);        /* ffff*p */

    ecp_nistz256_sqr_mont(res, p16);
    for (i = 0; i < 15; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(p32, res, p16);       /* ffffffff*p */

    ecp_nistz256_sqr_mont(res, p32);
    for (i = 0; i < 31; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, in);

    for (i = 0; i < 32 * 4; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p32);

    for (i = 0; i < 32; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p32);

    for (i = 0; i < 16; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p16);

    for (i = 0; i < 8; i++)
        ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p8);

    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p4);

    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, p2);

    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_sqr_mont(res, res);
    ecp_nistz256_mul_mont(res, res, in);

    memcpy(r, res, sizeof(res));
}

/* ecp_nistz256_bignum_to_field_elem copies the contents of |in| to |out| and
 * returns one if it fits. Otherwise it returns zero. */
static int ecp_nistz256_bignum_to_field_elem(BN_ULONG out[P256_LIMBS],
                                             const BIGNUM * in)
{
    if (in->top > P256_LIMBS)
        return 0;

    memset(out, 0, sizeof(BN_ULONG) * P256_LIMBS);
    memcpy(out, in->d, sizeof(BN_ULONG) * in->top);
    return 1;
}

/* r = sum(scalar[i]*point[i]) */
static void ecp_nistz256_windowed_mul(const EC_GROUP * group,
                                      P256_POINT * r,
                                      const BIGNUM ** scalar,
                                      const EC_POINT ** point,
                                      int num, BN_CTX * ctx)
{
    int i, j;
    unsigned int index;
    unsigned char (*p_str)[33] = NULL;
    const unsigned int window_size = 5;
    const unsigned int mask = (1 << (window_size + 1)) - 1;
    unsigned int wvalue;
    BN_ULONG tmp[P256_LIMBS];
    ALIGN32 P256_POINT h;
    const BIGNUM **scalars = NULL;
    P256_POINT(*table)[16] = NULL;
    void *table_storage = NULL;

    if ((table_storage =
         OPENSSL_malloc(num * 16 * sizeof(P256_POINT) + 64)) == NULL
        || (p_str =
            OPENSSL_malloc(num * 33 * sizeof(unsigned char))) == NULL
        || (scalars = OPENSSL_malloc(num * sizeof(BIGNUM *))) == NULL) {
        ECerr(EC_F_ECP_NISTZ256_WINDOWED_MUL, ERR_R_MALLOC_FAILURE);
        goto err;
    } else {
        table = (void *)ALIGNPTR(table_storage, 64);
    }

    for (i = 0; i < num; i++) {
        P256_POINT *row = table[i];

        if ((BN_num_bits(scalar[i]) > 256) || BN_is_negative(scalar[i])) {
            BIGNUM *mod;

            if ((mod = BN_CTX_get(ctx)) == NULL)
                goto err;
            if (!BN_nnmod(mod, scalar[i], &group->order, ctx)) {
                ECerr(EC_F_ECP_NISTZ256_WINDOWED_MUL, ERR_R_BN_LIB);
                goto err;
            }
            scalars[i] = mod;
        } else
            scalars[i] = scalar[i];

        for (j = 0; j < scalars[i]->top * BN_BYTES; j += BN_BYTES) {
            BN_ULONG d = scalars[i]->d[j / BN_BYTES];

            p_str[i][j + 0] = d & 0xff;
            p_str[i][j + 1] = (d >> 8) & 0xff;
            p_str[i][j + 2] = (d >> 16) & 0xff;
            p_str[i][j + 3] = (d >>= 24) & 0xff;
            if (BN_BYTES == 8) {
                d >>= 8;
                p_str[i][j + 4] = d & 0xff;
                p_str[i][j + 5] = (d >> 8) & 0xff;
                p_str[i][j + 6] = (d >> 16) & 0xff;
                p_str[i][j + 7] = (d >> 24) & 0xff;
            }
        }
        for (; j < 33; j++)
            p_str[i][j] = 0;

        /* table[0] is implicitly (0,0,0) (the point at infinity),
         * therefore it is not stored. All other values are actually
         * stored with an offset of -1 in table.
         */

        if (!ecp_nistz256_bignum_to_field_elem(row[1 - 1].X, &point[i]->X)
            || !ecp_nistz256_bignum_to_field_elem(row[1 - 1].Y, &point[i]->Y)
            || !ecp_nistz256_bignum_to_field_elem(row[1 - 1].Z, &point[i]->Z)) {
            ECerr(EC_F_ECP_NISTZ256_WINDOWED_MUL, EC_R_COORDINATES_OUT_OF_RANGE);
            goto err;
        }

        ecp_nistz256_point_double(&row[ 2 - 1], &row[ 1 - 1]);
        ecp_nistz256_point_add   (&row[ 3 - 1], &row[ 2 - 1], &row[1 - 1]);
        ecp_nistz256_point_double(&row[ 4 - 1], &row[ 2 - 1]);
        ecp_nistz256_point_double(&row[ 6 - 1], &row[ 3 - 1]);
        ecp_nistz256_point_double(&row[ 8 - 1], &row[ 4 - 1]);
        ecp_nistz256_point_double(&row[12 - 1], &row[ 6 - 1]);
        ecp_nistz256_point_add   (&row[ 5 - 1], &row[ 4 - 1], &row[1 - 1]);
        ecp_nistz256_point_add   (&row[ 7 - 1], &row[ 6 - 1], &row[1 - 1]);
        ecp_nistz256_point_add   (&row[ 9 - 1], &row[ 8 - 1], &row[1 - 1]);
        ecp_nistz256_point_add   (&row[13 - 1], &row[12 - 1], &row[1 - 1]);
        ecp_nistz256_point_double(&row[14 - 1], &row[ 7 - 1]);
        ecp_nistz256_point_double(&row[10 - 1], &row[ 5 - 1]);
        ecp_nistz256_point_add   (&row[15 - 1], &row[14 - 1], &row[1 - 1]);
        ecp_nistz256_point_add   (&row[11 - 1], &row[10 - 1], &row[1 - 1]);
        ecp_nistz256_point_add   (&row[16 - 1], &row[15 - 1], &row[1 - 1]);
    }

    index = 255;

    wvalue = p_str[0][(index - 1) / 8];
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;

    ecp_nistz256_select_w5(r, table[0], _booth_recode_w5(wvalue) >> 1);

    while (index >= 5) {
        for (i = (index == 255 ? 1 : 0); i < num; i++) {
            unsigned int off = (index - 1) / 8;

            wvalue = p_str[i][off] | p_str[i][off + 1] << 8;
            wvalue = (wvalue >> ((index - 1) % 8)) & mask;

            wvalue = _booth_recode_w5(wvalue);

            ecp_nistz256_select_w5(&h, table[i], wvalue >> 1);

            ecp_nistz256_neg(tmp, h.Y);
            copy_conditional(h.Y, tmp, (wvalue & 1));

            ecp_nistz256_point_add(r, r, &h);
        }

        index -= window_size;

        ecp_nistz256_point_double(r, r);
        ecp_nistz256_point_double(r, r);
        ecp_nistz256_point_double(r, r);
        ecp_nistz256_point_double(r, r);
        ecp_nistz256_point_double(r, r);
    }

    /* Final window */
    for (i = 0; i < num; i++) {
        wvalue = p_str[i][0];
        wvalue = (wvalue << 1) & mask;

        wvalue = _booth_recode_w5(wvalue);

        ecp_nistz256_select_w5(&h, table[i], wvalue >> 1);

        ecp_nistz256_neg(tmp, h.Y);
        copy_conditional(h.Y, tmp, wvalue & 1);

        ecp_nistz256_point_add(r, r, &h);
    }

err:
    if (table_storage)
        OPENSSL_free(table_storage);
    if (p_str)
        OPENSSL_free(p_str);
    if (scalars)
        OPENSSL_free(scalars);
}

/* Coordinates of G, for which we have precomputed tables */
const static BN_ULONG def_xG[P256_LIMBS] = {
    TOBN(0x79e730d4, 0x18a9143c), TOBN(0x75ba95fc, 0x5fedb601),
    TOBN(0x79fb732b, 0x77622510), TOBN(0x18905f76, 0xa53755c6)
};

const static BN_ULONG def_yG[P256_LIMBS] = {
    TOBN(0xddf25357, 0xce95560a), TOBN(0x8b4ab8e4, 0xba19e45c),
    TOBN(0xd2e88688, 0xdd21f325), TOBN(0x8571ff18, 0x25885d85)
};

/* ecp_nistz256_is_affine_G returns one if |generator| is the standard,
 * P-256 generator. */
static int ecp_nistz256_is_affine_G(const EC_POINT * generator)
{
    return (generator->X.top == P256_LIMBS) &&
        (generator->Y.top == P256_LIMBS) &&
        (generator->Z.top == (P256_LIMBS - P256_LIMBS / 8)) &&
        is_equal(generator->X.d, def_xG) &&
        is_equal(generator->Y.d, def_yG) && is_one(generator->Z.d);
}

static int ecp_nistz256_mult_precompute(EC_GROUP * group, BN_CTX * ctx)
{
    /* We precompute a table for a Booth encoded exponent (wNAF) based
     * computation. Each table holds 64 values for safe access, with an
     * implicit value of infinity at index zero. We use window of size 7,
     * and therefore require ceil(256/7) = 37 tables. */
    BIGNUM *order;
    EC_POINT *P = NULL, *T = NULL;
    const EC_POINT *generator;
    EC_PRE_COMP *pre_comp;
    int i, j, k, ret = 0;
    size_t w;

    PRECOMP256_ROW *preComputedTable = NULL;
    unsigned char *precomp_storage = NULL;

    /* if there is an old EC_PRE_COMP object, throw it away */
    EC_EX_DATA_free_data(&group->extra_data, ecp_nistz256_pre_comp_dup,
                         ecp_nistz256_pre_comp_free,
                         ecp_nistz256_pre_comp_clear_free);

    generator = EC_GROUP_get0_generator(group);
    if (generator == NULL) {
        ECerr(EC_F_ECP_NISTZ256_MULT_PRECOMPUTE, EC_R_UNDEFINED_GENERATOR);
        return 0;
    }

    if (ecp_nistz256_is_affine_G(generator)) {
        /* No need to calculate tables for the standard generator
         * because we have them statically. */
        return 1;
    }

    if ((pre_comp = ecp_nistz256_pre_comp_new(group)) == NULL)
        return 0;

    if (ctx == NULL) {
        ctx = BN_CTX_new();
        if (ctx == NULL)
            goto err;
    }

    BN_CTX_start(ctx);
    order = BN_CTX_get(ctx);

    if (order == NULL)
        goto err;

    if (!EC_GROUP_get_order(group, order, ctx))
        goto err;

    if (BN_is_zero(order)) {
        ECerr(EC_F_ECP_NISTZ256_MULT_PRECOMPUTE, EC_R_UNKNOWN_ORDER);
        goto err;
    }

    w = 7;

    if ((precomp_storage =
         OPENSSL_malloc(37 * 64 * sizeof(P256_POINT_AFFINE) + 64)) == NULL) {
        ECerr(EC_F_ECP_NISTZ256_MULT_PRECOMPUTE, ERR_R_MALLOC_FAILURE);
        goto err;
    } else {
        preComputedTable = (void *)ALIGNPTR(precomp_storage, 64);
    }

    P = EC_POINT_new(group);
    T = EC_POINT_new(group);

    /* The zero entry is implicitly infinity, and we skip it,
     * storing other values with -1 offset. */
    EC_POINT_copy(T, generator);

    for (k = 0; k < 64; k++) {
        EC_POINT_copy(P, T);
        for (j = 0; j < 37; j++) {
            /* It would be faster to use
             * ec_GFp_simple_points_make_affine and make multiple
             * points affine at the same time. */
            ec_GFp_simple_make_affine(group, P, ctx);
            ecp_nistz256_bignum_to_field_elem(preComputedTable[j]
                                              [k].X, &P->X);
            ecp_nistz256_bignum_to_field_elem(preComputedTable[j]
                                              [k].Y, &P->Y);
            for (i = 0; i < 7; i++)
                ec_GFp_simple_dbl(group, P, P, ctx);
        }
        ec_GFp_simple_add(group, T, T, generator, ctx);
    }

    pre_comp->group = group;
    pre_comp->w = w;
    pre_comp->precomp = preComputedTable;
    pre_comp->precomp_storage = precomp_storage;

    precomp_storage = NULL;

    if (!EC_EX_DATA_set_data(&group->extra_data, pre_comp,
                             ecp_nistz256_pre_comp_dup,
                             ecp_nistz256_pre_comp_free,
                             ecp_nistz256_pre_comp_clear_free)) {
        goto err;
    }

    pre_comp = NULL;

    ret = 1;

err:
    if (ctx != NULL)
        BN_CTX_end(ctx);
    if (pre_comp)
        ecp_nistz256_pre_comp_free(pre_comp);
    if (precomp_storage)
        OPENSSL_free(precomp_storage);
    if (P)
        EC_POINT_free(P);
    if (T)
        EC_POINT_free(T);
    return ret;
}

/*
 * Note that by default ECP_NISTZ256_AVX2 is undefined. While it's great
 * code processing 4 points in parallel, corresponding serial operation
 * is several times slower, because it uses 29x29=58-bit multiplication
 * as opposite to 64x64=128-bit in integer-only scalar case. As result
 * it doesn't provide *significant* performance improvement. Note that
 * just defining ECP_NISTZ256_AVX2 is not sufficient to make it work,
 * you'd need to compile even asm/ecp_nistz256-avx.pl module.
 */
#if defined(ECP_NISTZ256_AVX2)
# if !(defined(__x86_64) || defined(__x86_64__)) || \
       defined(_M_AMD64) || defined(_MX64)) || \
     !(defined(__GNUC__) || defined(_MSC_VER)) /* this is for ALIGN32 */
#  undef ECP_NISTZ256_AVX2
# else
/* Constant time access, loading four values, from four consecutive tables */
void ecp_nistz256_avx2_select_w7(P256_POINT_AFFINE * val,
                                 const P256_POINT_AFFINE * in_t, int index);
void ecp_nistz256_avx2_multi_select_w7(void *result, const void *in, int index0,
                                       int index1, int index2, int index3);
void ecp_nistz256_avx2_transpose_convert(void *RESULTx4, const void *in);
void ecp_nistz256_avx2_convert_transpose_back(void *result, const void *Ax4);
void ecp_nistz256_avx2_point_add_affine_x4(void *RESULTx4, const void *Ax4,
                                           const void *Bx4);
void ecp_nistz256_avx2_point_add_affines_x4(void *RESULTx4, const void *Ax4,
                                            const void *Bx4);
void ecp_nistz256_avx2_to_mont(void *RESULTx4, const void *Ax4);
void ecp_nistz256_avx2_from_mont(void *RESULTx4, const void *Ax4);
void ecp_nistz256_avx2_set1(void *RESULTx4);
int ecp_nistz_avx2_eligible(void);

static void booth_recode_w7(unsigned char *sign,
                            unsigned char *digit, unsigned char in)
{
    unsigned char s, d;

    s = ~((in >> 7) - 1);
    d = (1 << 8) - in - 1;
    d = (d & s) | (in & ~s);
    d = (d >> 1) + (d & 1);

    *sign = s & 1;
    *digit = d;
}

/* ecp_nistz256_avx2_mul_g performs multiplication by G, using only the
 * precomputed table. It does 4 affine point additions in parallel,
 * significantly speeding up point multiplication for a fixed value. */
static void ecp_nistz256_avx2_mul_g(P256_POINT * r,
                                    unsigned char p_str[33],
                                    const
                                    P256_POINT_AFFINE(*preComputedTable)[64])
{
    const unsigned int window_size = 7;
    const unsigned int mask = (1 << (window_size + 1)) - 1;
    unsigned int wvalue;
    /* Using 4 windows at a time */
    unsigned char sign0, digit0;
    unsigned char sign1, digit1;
    unsigned char sign2, digit2;
    unsigned char sign3, digit3;
    unsigned int index = 0;
    BN_ULONG tmp[P256_LIMBS];
    int i;

    ALIGN32 BN_ULONG aX4[4 * 9 * 3] = { 0 };
    ALIGN32 BN_ULONG bX4[4 * 9 * 2] = { 0 };
    ALIGN32 P256_POINT_AFFINE point_arr[P256_LIMBS];
    ALIGN32 P256_POINT res_point_arr[P256_LIMBS];

    /* Initial four windows */
    wvalue = *((u16 *) & p_str[0]);
    wvalue = (wvalue << 1) & mask;
    index += window_size;
    booth_recode_w7(&sign0, &digit0, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign1, &digit1, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign2, &digit2, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign3, &digit3, wvalue);

    ecp_nistz256_avx2_multi_select_w7(point_arr, preComputedTable[0],
                                      digit0, digit1, digit2, digit3);

    ecp_nistz256_neg(tmp, point_arr[0].Y);
    copy_conditional(point_arr[0].Y, tmp, sign0);
    ecp_nistz256_neg(tmp, point_arr[1].Y);
    copy_conditional(point_arr[1].Y, tmp, sign1);
    ecp_nistz256_neg(tmp, point_arr[2].Y);
    copy_conditional(point_arr[2].Y, tmp, sign2);
    ecp_nistz256_neg(tmp, point_arr[3].Y);
    copy_conditional(point_arr[3].Y, tmp, sign3);

    ecp_nistz256_avx2_transpose_convert(aX4, point_arr);
    ecp_nistz256_avx2_to_mont(aX4, aX4);
    ecp_nistz256_avx2_to_mont(&aX4[4 * 9], &aX4[4 * 9]);
    ecp_nistz256_avx2_set1(&aX4[4 * 9 * 2]);

    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign0, &digit0, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign1, &digit1, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign2, &digit2, wvalue);
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    index += window_size;
    booth_recode_w7(&sign3, &digit3, wvalue);

    ecp_nistz256_avx2_multi_select_w7(point_arr, preComputedTable[4 * 1],
                                      digit0, digit1, digit2, digit3);

    ecp_nistz256_neg(tmp, point_arr[0].Y);
    copy_conditional(point_arr[0].Y, tmp, sign0);
    ecp_nistz256_neg(tmp, point_arr[1].Y);
    copy_conditional(point_arr[1].Y, tmp, sign1);
    ecp_nistz256_neg(tmp, point_arr[2].Y);
    copy_conditional(point_arr[2].Y, tmp, sign2);
    ecp_nistz256_neg(tmp, point_arr[3].Y);
    copy_conditional(point_arr[3].Y, tmp, sign3);

    ecp_nistz256_avx2_transpose_convert(bX4, point_arr);
    ecp_nistz256_avx2_to_mont(bX4, bX4);
    ecp_nistz256_avx2_to_mont(&bX4[4 * 9], &bX4[4 * 9]);
    /* Optimized when both inputs are affine */
    ecp_nistz256_avx2_point_add_affines_x4(aX4, aX4, bX4);

    for (i = 2; i < 9; i++) {
        wvalue = *((u16 *) & p_str[(index - 1) / 8]);
        wvalue = (wvalue >> ((index - 1) % 8)) & mask;
        index += window_size;
        booth_recode_w7(&sign0, &digit0, wvalue);
        wvalue = *((u16 *) & p_str[(index - 1) / 8]);
        wvalue = (wvalue >> ((index - 1) % 8)) & mask;
        index += window_size;
        booth_recode_w7(&sign1, &digit1, wvalue);
        wvalue = *((u16 *) & p_str[(index - 1) / 8]);
        wvalue = (wvalue >> ((index - 1) % 8)) & mask;
        index += window_size;
        booth_recode_w7(&sign2, &digit2, wvalue);
        wvalue = *((u16 *) & p_str[(index - 1) / 8]);
        wvalue = (wvalue >> ((index - 1) % 8)) & mask;
        index += window_size;
        booth_recode_w7(&sign3, &digit3, wvalue);

        ecp_nistz256_avx2_multi_select_w7(point_arr,
                                          preComputedTable[4 * i],
                                          digit0, digit1, digit2, digit3);

        ecp_nistz256_neg(tmp, point_arr[0].Y);
        copy_conditional(point_arr[0].Y, tmp, sign0);
        ecp_nistz256_neg(tmp, point_arr[1].Y);
        copy_conditional(point_arr[1].Y, tmp, sign1);
        ecp_nistz256_neg(tmp, point_arr[2].Y);
        copy_conditional(point_arr[2].Y, tmp, sign2);
        ecp_nistz256_neg(tmp, point_arr[3].Y);
        copy_conditional(point_arr[3].Y, tmp, sign3);

        ecp_nistz256_avx2_transpose_convert(bX4, point_arr);
        ecp_nistz256_avx2_to_mont(bX4, bX4);
        ecp_nistz256_avx2_to_mont(&bX4[4 * 9], &bX4[4 * 9]);

        ecp_nistz256_avx2_point_add_affine_x4(aX4, aX4, bX4);
    }

    ecp_nistz256_avx2_from_mont(&aX4[4 * 9 * 0], &aX4[4 * 9 * 0]);
    ecp_nistz256_avx2_from_mont(&aX4[4 * 9 * 1], &aX4[4 * 9 * 1]);
    ecp_nistz256_avx2_from_mont(&aX4[4 * 9 * 2], &aX4[4 * 9 * 2]);

    ecp_nistz256_avx2_convert_transpose_back(res_point_arr, aX4);
    /* Last window is performed serially */
    wvalue = *((u16 *) & p_str[(index - 1) / 8]);
    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
    booth_recode_w7(&sign0, &digit0, wvalue);
    ecp_nistz256_avx2_select_w7((P256_POINT_AFFINE *) r,
                                preComputedTable[36], digit0);
    ecp_nistz256_neg(tmp, r->Y);
    copy_conditional(r->Y, tmp, sign0);
    memcpy(r->Z, ONE, sizeof(ONE));
    /* Sum the four windows */
    ecp_nistz256_point_add(r, r, &res_point_arr[0]);
    ecp_nistz256_point_add(r, r, &res_point_arr[1]);
    ecp_nistz256_point_add(r, r, &res_point_arr[2]);
    ecp_nistz256_point_add(r, r, &res_point_arr[3]);
}
# endif
#endif

static int ecp_nistz256_set_from_affine(EC_POINT * out, const EC_GROUP * group,
                                        const P256_POINT_AFFINE * in,
                                        BN_CTX * ctx)
{
    BIGNUM x, y;
    BN_ULONG d_x[P256_LIMBS], d_y[P256_LIMBS];
    int ret = 0;

    memcpy(d_x, in->X, sizeof(d_x));
    x.d = d_x;
    x.dmax = x.top = P256_LIMBS;
    x.neg = 0;
    x.flags = BN_FLG_STATIC_DATA;

    memcpy(d_y, in->Y, sizeof(d_y));
    y.d = d_y;
    y.dmax = y.top = P256_LIMBS;
    y.neg = 0;
    y.flags = BN_FLG_STATIC_DATA;

    ret = EC_POINT_set_affine_coordinates_GFp(group, out, &x, &y, ctx);

    return ret;
}

/* r = scalar*G + sum(scalars[i]*points[i]) */
static int ecp_nistz256_points_mul(const EC_GROUP * group,
                                   EC_POINT * r,
                                   const BIGNUM * scalar,
                                   size_t num,
                                   const EC_POINT * points[],
                                   const BIGNUM * scalars[], BN_CTX * ctx)
{
    int i = 0, ret = 0, no_precomp_for_generator = 0, p_is_infinity = 0;
    size_t j;
    unsigned char p_str[33] = { 0 };
    const PRECOMP256_ROW *preComputedTable = NULL;
    const EC_PRE_COMP *pre_comp = NULL;
    const EC_POINT *generator = NULL;
    unsigned int index = 0;
    const unsigned int window_size = 7;
    const unsigned int mask = (1 << (window_size + 1)) - 1;
    unsigned int wvalue;
    ALIGN32 union {
        P256_POINT p;
        P256_POINT_AFFINE a;
    } t, p;
    BIGNUM *tmp_scalar;

    if (group->meth != r->meth) {
        ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, EC_R_INCOMPATIBLE_OBJECTS);
        return 0;
    }
    if ((scalar == NULL) && (num == 0))
        return EC_POINT_set_to_infinity(group, r);

    for (j = 0; j < num; j++) {
        if (group->meth != points[j]->meth) {
            ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, EC_R_INCOMPATIBLE_OBJECTS);
            return 0;
        }
    }

    /* Need 256 bits for space for all coordinates. */
    bn_wexpand(&r->X, P256_LIMBS);
    bn_wexpand(&r->Y, P256_LIMBS);
    bn_wexpand(&r->Z, P256_LIMBS);
    r->X.top = P256_LIMBS;
    r->Y.top = P256_LIMBS;
    r->Z.top = P256_LIMBS;

    if (scalar) {
        generator = EC_GROUP_get0_generator(group);
        if (generator == NULL) {
            ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, EC_R_UNDEFINED_GENERATOR);
            goto err;
        }

        /* look if we can use precomputed multiples of generator */
        pre_comp =
            EC_EX_DATA_get_data(group->extra_data, ecp_nistz256_pre_comp_dup,
                                ecp_nistz256_pre_comp_free,
                                ecp_nistz256_pre_comp_clear_free);

        if (pre_comp) {
            /* If there is a precomputed table for the generator,
             * check that it was generated with the same
             * generator. */
            EC_POINT *pre_comp_generator = EC_POINT_new(group);
            if (pre_comp_generator == NULL)
                goto err;

            if (!ecp_nistz256_set_from_affine
                (pre_comp_generator, group, pre_comp->precomp[0], ctx))
                goto err;

            if (0 == EC_POINT_cmp(group, generator, pre_comp_generator, ctx))
                preComputedTable = (const PRECOMP256_ROW *)pre_comp->precomp;

            EC_POINT_free(pre_comp_generator);
        }

        if (preComputedTable == NULL && ecp_nistz256_is_affine_G(generator)) {
            /* If there is no precomputed data, but the generator
             * is the default, a hardcoded table of precomputed
             * data is used. This is because applications, such as
             * Apache, do not use EC_KEY_precompute_mult. */
            preComputedTable = (const PRECOMP256_ROW *)ecp_nistz256_precomputed;
        }

        if (preComputedTable) {
            if ((BN_num_bits(scalar) > 256)
                || BN_is_negative(scalar)) {
                if ((tmp_scalar = BN_CTX_get(ctx)) == NULL)
                    goto err;

                if (!BN_nnmod(tmp_scalar, scalar, &group->order, ctx)) {
                    ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, ERR_R_BN_LIB);
                    goto err;
                }
                scalar = tmp_scalar;
            }

            for (i = 0; i < scalar->top * BN_BYTES; i += BN_BYTES) {
                BN_ULONG d = scalar->d[i / BN_BYTES];

                p_str[i + 0] = d & 0xff;
                p_str[i + 1] = (d >> 8) & 0xff;
                p_str[i + 2] = (d >> 16) & 0xff;
                p_str[i + 3] = (d >>= 24) & 0xff;
                if (BN_BYTES == 8) {
                    d >>= 8;
                    p_str[i + 4] = d & 0xff;
                    p_str[i + 5] = (d >> 8) & 0xff;
                    p_str[i + 6] = (d >> 16) & 0xff;
                    p_str[i + 7] = (d >> 24) & 0xff;
                }
            }

            for (; i < 33; i++)
                p_str[i] = 0;

#if defined(ECP_NISTZ256_AVX2)
            if (ecp_nistz_avx2_eligible()) {
                ecp_nistz256_avx2_mul_g(&p.p, p_str, preComputedTable);
            } else
#endif
            {
                /* First window */
                wvalue = (p_str[0] << 1) & mask;
                index += window_size;

                wvalue = _booth_recode_w7(wvalue);

                ecp_nistz256_select_w7(&p.a, preComputedTable[0], wvalue >> 1);

                ecp_nistz256_neg(p.p.Z, p.p.Y);
                copy_conditional(p.p.Y, p.p.Z, wvalue & 1);

                memcpy(p.p.Z, ONE, sizeof(ONE));

                for (i = 1; i < 37; i++) {
                    unsigned int off = (index - 1) / 8;
                    wvalue = p_str[off] | p_str[off + 1] << 8;
                    wvalue = (wvalue >> ((index - 1) % 8)) & mask;
                    index += window_size;

                    wvalue = _booth_recode_w7(wvalue);

                    ecp_nistz256_select_w7(&t.a,
                                           preComputedTable[i], wvalue >> 1);

                    ecp_nistz256_neg(t.p.Z, t.a.Y);
                    copy_conditional(t.a.Y, t.p.Z, wvalue & 1);

                    ecp_nistz256_point_add_affine(&p.p, &p.p, &t.a);
                }
            }
        } else {
            p_is_infinity = 1;
            no_precomp_for_generator = 1;
        }
    } else
        p_is_infinity = 1;

    if (no_precomp_for_generator) {
        /* Without a precomputed table for the generator, it has to be
         * handled like a normal point. */
        const BIGNUM **new_scalars;
        const EC_POINT **new_points;

        new_scalars = OPENSSL_malloc((num + 1) * sizeof(BIGNUM *));
        if (!new_scalars) {
            ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, ERR_R_MALLOC_FAILURE);
            return 0;
        }

        new_points = OPENSSL_malloc((num + 1) * sizeof(EC_POINT *));
        if (!new_points) {
            OPENSSL_free(new_scalars);
            ECerr(EC_F_ECP_NISTZ256_POINTS_MUL, ERR_R_MALLOC_FAILURE);
            return 0;
        }

        memcpy(new_scalars, scalars, num * sizeof(BIGNUM *));
        new_scalars[num] = scalar;
        memcpy(new_points, points, num * sizeof(EC_POINT *));
        new_points[num] = generator;

        scalars = new_scalars;
        points = new_points;
        num++;
    }

    if (num) {
        P256_POINT *out = &t.p;
        if (p_is_infinity)
            out = &p.p;

        ecp_nistz256_windowed_mul(group, out, scalars, points, num, ctx);

        if (!p_is_infinity)
            ecp_nistz256_point_add(&p.p, &p.p, out);
    }

    if (no_precomp_for_generator) {
        OPENSSL_free(points);
        OPENSSL_free(scalars);
    }

    memcpy(r->X.d, p.p.X, sizeof(p.p.X));
    memcpy(r->Y.d, p.p.Y, sizeof(p.p.Y));
    memcpy(r->Z.d, p.p.Z, sizeof(p.p.Z));
    bn_correct_top(&r->X);
    bn_correct_top(&r->Y);
    bn_correct_top(&r->Z);

    ret = 1;

err:
    return ret;
}

static int ecp_nistz256_get_affine(const EC_GROUP * group,
                                   const EC_POINT * point,
                                   BIGNUM * x, BIGNUM * y, BN_CTX * ctx)
{
    BN_ULONG z_inv2[P256_LIMBS];
    BN_ULONG z_inv3[P256_LIMBS];
    BN_ULONG x_aff[P256_LIMBS];
    BN_ULONG y_aff[P256_LIMBS];
    BN_ULONG point_x[P256_LIMBS], point_y[P256_LIMBS], point_z[P256_LIMBS];

    if (EC_POINT_is_at_infinity(group, point)) {
        ECerr(EC_F_ECP_NISTZ256_GET_AFFINE, EC_R_POINT_AT_INFINITY);
        return 0;
    }

    if (!ecp_nistz256_bignum_to_field_elem(point_x, &point->X) ||
        !ecp_nistz256_bignum_to_field_elem(point_y, &point->Y) ||
        !ecp_nistz256_bignum_to_field_elem(point_z, &point->Z)) {
        ECerr(EC_F_ECP_NISTZ256_GET_AFFINE, EC_R_COORDINATES_OUT_OF_RANGE);
        return 0;
    }

    ecp_nistz256_mod_inverse(z_inv3, point_z);
    ecp_nistz256_sqr_mont(z_inv2, z_inv3);
    ecp_nistz256_mul_mont(x_aff, z_inv2, point_x);

    if (x != NULL) {
        bn_wexpand(x, P256_LIMBS);
        x->top = P256_LIMBS;
        ecp_nistz256_from_mont(x->d, x_aff);
        bn_correct_top(x);
    }

    if (y != NULL) {
        ecp_nistz256_mul_mont(z_inv3, z_inv3, z_inv2);
        ecp_nistz256_mul_mont(y_aff, z_inv3, point_y);
        bn_wexpand(y, P256_LIMBS);
        y->top = P256_LIMBS;
        ecp_nistz256_from_mont(y->d, y_aff);
        bn_correct_top(y);
    }

    return 1;
}

static EC_PRE_COMP *ecp_nistz256_pre_comp_new(const EC_GROUP * group)
{
    EC_PRE_COMP *ret = NULL;

    if (!group)
        return NULL;

    ret = (EC_PRE_COMP *) OPENSSL_malloc(sizeof(EC_PRE_COMP));

    if (!ret) {
        ECerr(EC_F_ECP_NISTZ256_PRE_COMP_NEW, ERR_R_MALLOC_FAILURE);
        return ret;
    }

    ret->group = group;
    ret->w = 6;                 /* default */
    ret->precomp = NULL;
    ret->precomp_storage = NULL;
    ret->references = 1;
    return ret;
}

static void *ecp_nistz256_pre_comp_dup(void *src_)
{
    EC_PRE_COMP *src = src_;

    /* no need to actually copy, these objects never change! */
    CRYPTO_add(&src->references, 1, CRYPTO_LOCK_EC_PRE_COMP);

    return src_;
}

static void ecp_nistz256_pre_comp_free(void *pre_)
{
    int i;
    EC_PRE_COMP *pre = pre_;

    if (!pre)
        return;

    i = CRYPTO_add(&pre->references, -1, CRYPTO_LOCK_EC_PRE_COMP);
    if (i > 0)
        return;

    if (pre->precomp_storage)
        OPENSSL_free(pre->precomp_storage);

    OPENSSL_free(pre);
}

static void ecp_nistz256_pre_comp_clear_free(void *pre_)
{
    int i;
    EC_PRE_COMP *pre = pre_;

    if (!pre)
        return;

    i = CRYPTO_add(&pre->references, -1, CRYPTO_LOCK_EC_PRE_COMP);
    if (i > 0)
        return;

    if (pre->precomp_storage) {
        OPENSSL_cleanse(pre->precomp,
                        32 * sizeof(unsigned char) * (1 << pre->w) * 2 * 37);
        OPENSSL_free(pre->precomp_storage);
    }
    OPENSSL_cleanse(pre, sizeof *pre);
    OPENSSL_free(pre);
}

static int ecp_nistz256_window_have_precompute_mult(const EC_GROUP * group)
{
    /* There is a hard-coded table for the default generator. */
    const EC_POINT *generator = EC_GROUP_get0_generator(group);
    if (generator != NULL && ecp_nistz256_is_affine_G(generator)) {
        /* There is a hard-coded table for the default generator. */
        return 1;
    }

    return EC_EX_DATA_get_data(group->extra_data, ecp_nistz256_pre_comp_dup,
                               ecp_nistz256_pre_comp_free,
                               ecp_nistz256_pre_comp_clear_free) != NULL;
}

const EC_METHOD *EC_GFp_nistz256_method(void)
{
    static const EC_METHOD ret = {
        EC_FLAGS_DEFAULT_OCT,
        NID_X9_62_prime_field,
        ec_GFp_mont_group_init,
        ec_GFp_mont_group_finish,
        ec_GFp_mont_group_clear_finish,
        ec_GFp_mont_group_copy,
        ec_GFp_mont_group_set_curve,
        ec_GFp_simple_group_get_curve,
        ec_GFp_simple_group_get_degree,
        ec_GFp_simple_group_check_discriminant,
        ec_GFp_simple_point_init,
        ec_GFp_simple_point_finish,
        ec_GFp_simple_point_clear_finish,
        ec_GFp_simple_point_copy,
        ec_GFp_simple_point_set_to_infinity,
        ec_GFp_simple_set_Jprojective_coordinates_GFp,
        ec_GFp_simple_get_Jprojective_coordinates_GFp,
        ec_GFp_simple_point_set_affine_coordinates,
        ecp_nistz256_get_affine,
        0, 0, 0,
        ec_GFp_simple_add,
        ec_GFp_simple_dbl,
        ec_GFp_simple_invert,
        ec_GFp_simple_is_at_infinity,
        ec_GFp_simple_is_on_curve,
        ec_GFp_simple_cmp,
        ec_GFp_simple_make_affine,
        ec_GFp_simple_points_make_affine,
        ecp_nistz256_points_mul,                    /* mul */
        ecp_nistz256_mult_precompute,               /* precompute_mult */
        ecp_nistz256_window_have_precompute_mult,   /* have_precompute_mult */
        ec_GFp_mont_field_mul,
        ec_GFp_mont_field_sqr,
        0,                                          /* field_div */
        ec_GFp_mont_field_encode,
        ec_GFp_mont_field_decode,
        ec_GFp_mont_field_set_to_one
    };

    return &ret;
}